Future communication systems (e.g., wireless wireless/cellular communication systems) are expected to require, among other things, increased coverage and/or support of higher data rates. A method to enhance coverage and data rate is to use some sort of forwarding node (FN) such as repeater or relay. The benefit from repeaters and relays stems primarily from their facilitating the splitting of a long distance link in to two links (or hops). This splitting allows increased data rate on each link as well as end-to-end total data rate.
The distinction between the repeaters and relays is not always entirely clear cut. Often, with a repeater, one means a simple low processing node that receives a signal, amplifies it and sends it out. Such nodes are referred to as Amplify and Forward (AF) nodes.
A relay, on the other hand, is normally considered a bit more advanced and complex than a repeater. A relay typically receives a signal, demodulates and forward error correction (FEC) decodes the signal, and then selects resource for retransmitting the signal. We will use the term Decode and Forward (DF) node for a node that demodulates and decodes a received signal prior to retransmitting the data included in the signal. Some relays may forward an estimate of its received signal to the destination. This type of relay is referred to as an Estimate and Forward (EF) node. To complicate things a bit, the term relay is often used broadly to encompass any node that forwards any piece of received information.
There are many different types of repeaters, such as frequency translating repeaters (FTR) and on-frequency repeaters (OFR). The so called on-frequency repeater is advantageous in that it avoids throughput loss that arises in many schemes where the forwarding node can not receive and transmit at the same time and on the same frequency. This loss is here denoted as duplex loss. Frequency-translating repeaters and relays generally suffer from duplex loss due to the normal operation of receiving and transmitting in different timeslots.
The on-frequency operation, which provides concurrent reception and transmission, is often achieved by high input-output antenna isolation and active self-interference cancellation. The later means that the repeater internally cancels the repeater output signal that is received by the repeater (i.e., the repeater feedback signal).
A shortcoming of using an on-frequency repeater is that the gain by which the repeater can amplify the incoming signal is limited. The maximum gain is mainly determined by the isolation between the repeater output and the repeater input and other means to cancel the feedback, such as any attenuation offered by internal interference cancellation. While several mechanisms have been suggested to further increase the output to input isolation (e.g., beamforming and MIMO techniques), it is likely that there will be situations when one would like to have larger on-frequency repeater output-to-input isolation than can be achieved using conventional means.
What is desired, therefore, are systems and methods for achieving larger output-to-input isolation than can be achieved using conventional means.